Dynamic rheology studies of in situ polymerization process of polyacrylamide-cellulose nanocrystal composite hydrogels

A series of dynamic small-amplitude oscillatory shear experiments for in situ polymerization process of polyacrylamide-cellulose nanocrystal (PAM-CNC) nano-composite hydrogels were performed to investigate the relationship between rheological properties and synthesis parameters including chemical cross-linker concentration, polymerization temperature, initiator concentration, and CNC aspect ratios. The results showed that CNCs accelerated the onset of gelation (t(onset)) and acted as a multifunctional cross-linker during the gelation reaction. The composite hydrogels exhibited enhanced steady-state elastic modulus (G') and plateau loss factor (t(an delta)) compared to these of the pure PAM hydrogels, indicating that adding CNCs not only reinforced but also toughened PAM hydrogels. (G(infinity)') and the effective network junction density (N) increased with increased cross-linker concentration, polymerization temperature, and CNC aspect ratios, but decreased with increased initiator concentration. The changes of plateau t(an delta) were opposite to that of G(infinity)'. The sol-gel transition kinetics of PAM-CNC hydrogels accelerated with increased cross-linker concentration and polymerization temperature and, however, reached optimization at 0.25 wt% of initiator concentration. CNCs with lower aspect ratios promoted t(onset) and the sol-gel transition of PAM-CNC hydrogels, suggesting the fact that CNCs with lower aspect ratios further facilitated the formation of network of PAM-CNC nanocomposite hydrogels.